1. Field of the Invention
The present invention relates to a memory, more specifically, relates to magnetic random access memory (MRAM).
2. Description of Related Art
Magnetic memory, for example, magnetic random access memory (MRAM), is a non-volatile memory with the advantages of non-volatility, high density, high read and write speed, and anti-radiation. When writing data, the commonly used method is that: a cell unit is selected by the intersection of the induced magnetic fields of two current lines (Write Bit Line and Write Word Line). The magnetoresistance value of the magnetic memory cell is changed by changing the magnetization direction of the magnetic material in the memory layer. When reading stored data, a sense amplifier provides a current to the selected magnetic memory cell unit, and the resistance value of the magnetic memory cell unit is read to determine the digital value of the stored data.
The magnetic memory cell unit is a stacking structure of a plurality of layers of magnetic metal material. The structure thereof is a stack of soft magnetic layer, tunneling barrier layer, hard magnetic layer and nonmagnetic conductive layer. Through the magnetization directions of the ferromagnetic materials on the two sides of the tunneling barrier layer being parallel or anti-parallel to determine storing the state of “1” or “0”.
FIG. 1 schematically illustrates a basic structure of a magnetic memory cell. With reference to FIG. 1, to write into a magnetic memory cell, the current lines 100, 102 are also needed. The current lines 100, 102 intersect with each other, and an appropriate current is introduced to the current lines 100, 102. According to the operation method thereof, the current lines 100, 102 for example may also be called bit line and word line. When current is passed to the two lines, magnetic fields of two directions are generated, so as to obtain the needed magnetic field and direction to apply on a magnetic memory cell 104. The magnetic memory cell 104 is a stacking structure, including a magnetic pinned layer with a fixed magnetization, or total magnetic moment at a predetermined direction. The angle difference of magnetizations between the magnetic free layer and the magnetic pinned layer generates different quantity of magnetoresistance, which is used to read data. In addition, when writing data in, a write magnetic field may also be applied to determine the magnetization direction of the magnetic free layer. Through output electrodes 106, 108, the data stored in the memory cell can be read out. The operation detail of the magnetic memory may be known by those with ordinary skill in the arts, therefore will not be further addressed.
FIG. 2 schematically illustrates the mechanism of a magnetic memory. With reference to FIG. 2, the magnetic pinned layer 104a has the magnetic moment direction 107 with a fixed direction. The magnetic free layer 104c is above the magnetic pinned layer 104a, there is a tunneling barrier layer 104b in between for separation. The magnetic free layer 104c has a magnetic moment direction 108a or 108b. Since the magnetic moment direction 107 is parallel to the magnetic moment direction 108a, the generated magnetoresistance thereof for example represents the data of “0”. On the contrary, the magnetic moment direction 107 is anti-parallel to the magnetic moment direction 108b, and the generated magnetoresistance thereof for example represents the data of “1”.
The magnetic free layer 104c of above FIG. 2 is a single layer structure, and therefore data error is likely to occur in operation. U.S. Pat. No. 6,545,906 provided that in order to reduce the interference when the adjacent cell unit is written data, in the free layer, single layer ferromagnetic material is replaced by ferromagnetic/non-ferromagnetic/ferromagnetic three layers structure. The free layer is a synthetic anti-ferromagnetic structure, the top and bottom magnetic layers respectively have a magnetization. In order to reduce the interference when the adjacent cell unit is written data, the ferromagnetic layers of top and bottom layers are arranged in anti-parallel to form closed magnetic line of force. In addition, in association with toggle operation mode, the write bit line and the write word line has an including angle of 45 degrees with the magnetic easy axis of the free layer, and the provided currents are to be applied with a specific sequence. Such method can effectively solve the interference problem.
However, people with ordinary skill in the arts are still actively researching and developing magnetic memory design, and also expect to reduce the size of element and also increase the integration of elements in the same time.